Implement Scan with a type switch

This is just a minor (and a bit opinionated) tweak in Scan. While using
the generated Scan method, realized that what's being returned from any
driver I've stumbled upon is actually []byte instead of string, thus
always causing a one failed type assertion on Scan.

golden_test.go

@@ -1682,14 +1682,16 @@ func (i *Prime) Scan(value interface{}) error {
 		return nil
 	}
 
-	str, ok := value.(string)
+	var str string
-	if !ok {
+	switch v := value.(type) {
-		bytes, ok := value.([]byte)
+	case []byte:
-		if !ok {
+		str = string(b)
-			return fmt.Errorf("value is not a byte slice")
+	case string:
-		}
+		str = v
-
+	case fmt.Stringer:
-		str = string(bytes[:])
+		str = v.String()
+	default:
+		return fmt.Errorf("invalid value of Prime: %[1]T(%[1]v)", value)
 	}
 
 	val, err := PrimeString(str)
@@ -1867,14 +1869,16 @@ func (i *Prime) Scan(value interface{}) error {
 		return nil
 	}
 
-	str, ok := value.(string)
+	var str string
-	if !ok {
+	switch v := value.(type) {
-		bytes, ok := value.([]byte)
+	case []byte:
-		if !ok {
+		str = string(b)
-			return fmt.Errorf("value is not a byte slice")
+	case string:
-		}
+		str = v
-
+	case fmt.Stringer:
-		str = string(bytes[:])
+		str = v.String()
+	default:
+		return fmt.Errorf("invalid value of Prime: %[1]T(%[1]v)", value)
 	}
 
 	val, err := PrimeString(str)

sql.go

@@ -12,21 +12,23 @@ const scanMethod = `func (i *%[1]s) Scan(value interface{}) error {
 		return nil
 	}
 
-	str, ok := value.(string)
+	var str string
-	if !ok {
+	switch v := value.(type) {
-		bytes, ok := value.([]byte)
+	case []byte:
-		if !ok {
+		str = string(b)
-			return fmt.Errorf("value is not a byte slice")
+	case string:
-		}
+		str = v
-
+	case fmt.Stringer:
-		str = string(bytes[:])
+		str = v.String()
+	default:
+		return fmt.Errorf("invalid value of %[1]s: %%[1]T(%%[1]v)", value)
 	}
 
 	val, err := %[1]sString(str)
 	if err != nil {
 		return err
 	}
-	
+
 	*i = val
 	return nil
 }